Modern pacemakers are typically made up of three primary components: a control circuit, an output circuit, and a power source. The control circuit determines the rate, synchronization, pulse width, and output voltage of heart stimulating pulses that are generated by the pacemaker. The control circuit may also perform diagnostic functions which are necessary to the safe operation of the pacer. For example, the circuit may provide backup functions in the event of a failure that could result in dangerous overstimulation or a potentially fatal non-stimulation of the heart. The control circuit is therefore an essential operating component which must be fully operational throughout the life of the pacer.
The power source, typically a lithium iodide battery, is chosen for its high energy density and its low self discharge characteristics. The lithium iodide battery has an internal impedance that increases as current is drawn from it. Therefore, if a constant current is drawn from the battery, the output voltage declines with time even though the unloaded voltage of the battery is very nearly unchanged. The internal impedance makes it impossible to deliver the amount of current required to stimulate the heart directly from the battery. Consequently, the output circuit uses an output capacitor which is charged between pace events and, upon command from the control circuit, discharged for a fixed length of time (usually 0.3 to 1.5 milliseconds) into the heart to stimulate it.
In prior art pacemakers, the battery is connected at all times to both the control circuit and the output circuit. The control circuit, which typically comprises digital circuitry and today may be microprocessor based, draws relatively little power from the battery, but must be continuously supplied. The output circuit, on the other hand, requires a relatively large amount of power but draws it most heavily during a peak demand period after the heart has been paced. The large power drain after pacing is required to recharge the output capacitor as quickly as possible.
As the output capacitor is recharged, there is a drop in battery voltage due to the charging current flowing through the battery impedance. This voltage drop is negligible when the battery is fresh. However, as the battery ages its impedance increases and the voltage drop increases proportionately. Thus, the battery voltage to the control circuit may drop below a minimum operating level when the output capacitor is being recharged. This temporary drop in voltage can cause a potentially dangerous intermittent malfunction of the control circuit and corresponding erratic operation of the pacer. Prior art pacemakers must be removed and replaced before such low voltage malfunctions occur, even though the battery may still be capable of supplying energy sufficient to stimulate the heart.
It can be seen, therefore, that the reliability of prior art pacemakers is compromised at the end of their lifetime. The decreased reliability is not based on the failure of the pacemaker to perform its primary function, i.e. to stimulate the heart, but because of fluctuations in battery voltage below the minimum allowable operating voltage of the control circuit. Thus, pacemaker reliability can be increased by reducing the likelihood that the voltage powering the control circuit will fall below its minimum allowable operating voltage.
Furthermore, the longevity of prior art pacemakers is reduced because transient low voltage conditions require removal of such pacemakers even though the average output voltage of the battery may be sufficient to supply the average demand of the control circuit and the output circuit. Thus, prior art pacers are inefficient in operation. Since longevity is also an important consideration in implantable pacers, it is desirable to increase the useful life of a pacer and its battery as much as possible by efficiently utilizing battery power.